This is a bugfix for SelectNodeTo. In certain situations, we could be

selecting a node and use a mix of getTargetNode() and SelectNodeTo.  Because
SelectNodeTo didn't check the CSE maps for a preexisting node and didn't insert
its result into the CSE maps, we would sometimes miss a CSE opportunity.

This is extremely rare, but worth fixing for completeness.

llvm-svn: 24565